Fraction collectors are widely used in many applications, such as in the field of liquid chromatography. A fraction collector is used for dispensing a flow of a liquid to a number of receptacles. The receptacles are typically constituted as test tubes mounted in a rack or as recesses formed in a plate. Two main working principles can be distinguished for fraction collectors: the rotatable collector wherein the receptacles are fed towards a dispensing means by a rotating movement, and the X-Y collector wherein the receptacles are fed towards a dispensing means by linear movements in one or two directions. Of course, these principles are the same as those for fraction collectors wherein the dispensing means is moving while the receptacles are at rest.
Regardless of the type of receptacle or fraction collector working principle, there is always a distance between each separate receptacle. Therefore, when switching from one receptacle to the next, a spillage of the dispensed liquid is likely to occur, especially in a case were the liquid flow is essentially continuous. There are numerous reasons for why such spillage is not desired: it could contain valuable substances, it could be a potential health hazard and the working area becomes messy.
Methods for avoiding the spillage are known. For example, in U.S. Pat. No. 4,077,444 to Gilson et al. there is described a valve and a valve operator that are used to discontinue a liquid flow through a dispensing tube in order to prevent spillage from the tube as it moves between positions. However, in certain applications, such as high precision liquid chromatography, interruption of the liquid flow during a hold time is a disadvantage. The performance of the liquid chromatography system is negatively affected due to the occurrence of diffusion of the components in the liquid volume held in the tubing near the dispensing means during the hold time.
It is also known to use a shunt valve to convey the liquid flow to waste during the receptacle change. This method has obviously the disadvantage that valuable substances may be present in the wasted flow, and consequently are lost.
In JP-A-01068657 a method and device for dispensing column chromatography eluate is described and in JP-A-59026058 a dispensing method and dispensing device are described.
U.S. Pat. No. 6,610,208 discloses a fraction collector that comprises a liquid holding means provided for avoiding loss of liquid during fraction collecting when switching from one collecting receptacle (3) to the next (3′) in the fraction collector (1), said liquid holding means comprises an expandable chamber that is actuated during the movement from one receptacle to another in order to stop the output flow from the dispensing nozzle.
However, in some situations when the expandable chamber is actuated to accumulate the flow during the movement, the rate of accumulation might be too high, e.g. due to compensation for errors between actual and reported flow. This may cause air to be drawn from the dispensing nozzle and into the chamber. Thereafter, when the expandable chamber is actuated to dispense the accumulated flow, the mixture of air and fluid in the chamber that is forced out through the nozzle may form bubbles which pops and sends small drops in all directions. These small drops may contaminate the samples in adjacent receptacles and will also accumulate on the sensors and cause malfunction.
Moreover, in fraction collectors wherein the dispensing nozzle is moving while the receptacles are at rest, acceleration of the dispensing nozzle when moving between two receptacles while a drop is still present at the dispensing nozzle may lead to detachment of the drop.